Nuclear magnetic resonance measuring and control device



Nov. 15, 1960- c. w. PINKLEY 2,960,650

NUCLEAR MAGNETIC RESONANCE MEASURING AND CONTROL DEVICE Filed Jan. 20, 1958 2 Sheets-Sheet i READOUT l INVENTOR 5 9 4 2 2 2 2 2 3 2 W e 2 m n m D SP T uM m m A H h PS 5 2 BISTABLE M R M "m P m m H S 5 F 3 U Y 2 N 3 e m M 8 R A.. L T U U SM 0 2 S 4 O R l O 3 N 0 8 M 0 0 8 T 3 M 3 2, m C E R% 9 5 RR R EE C GP E mm D R 5 0 L \4 u G QM P E l RT DM FN A O A 4 C Nov. 15, 1960 w PNKLEY 2,900,050

NUCLEAR MAGNETIC RESONANCE MEASURING AND CONTROL DEVICE 'iled Jan. 20, 1958 2 Sheets-Sheet 2 FIELD STRENGTH- I 35 INVENTOR NUCLEAR MAGNETIC RESCNANCE MEASURING AND CONTROL DEVICE Clyde W. Pinkley, Columbus, Ohio, assighor to Industrial Nucleonics Corporation, a corporation of Ohio Filed Jan. 20, 1958, 'Ser. No. 710,043

7 Claims. (Cl. 324--.5)

This invention relates to nuclear magnetic resonance measuring and control devices, and in particular to new and improved apparatus for automatically controlling the relative relationship between the magnetic and radio-frequency fields of such devices so as to satisfy the requirements for nuclear resonance.

It is well known in the prior art relating to nuclear physics that many atomic nuclei possess magnetic moment and nuclear momentum or spin. A nucleus having these characteristics displays gyroscopi'c effects and is therefore often considered analogous to a spinning gyroscope having a magnet positioned along its axis.

When such nuclei are subjected to a unidirectional magnetic field, the spinning nuclei initially tend to precess around an axis parallel to the magnetic field. After a period of time, damping forces suppress the nuclear precession enabling the nuclear moments to line up with the magnetic field. In the event the polarized nuclei are subjected to a radio-frequency field at right angles to the magnetic field, nuclear precession is again initiated.

Prior investigations have studied the gyroscopic properties of nuclei by subjecting an element to a magnetic field produced by a permanent magnet and simultaneously irradiating the element with radio-frequency electromagnetic energy emanating from a tank coil. a

When the frequency of the radio-frequency source resonates with the frequency of nuclear precession, the

spinning nuclei absorb a maximum amount of energy from the radio-frequency field thereby loading the tank circuit. It has been determined that the resonant frequency of nuclear precession varies for different elements and for different values of the polarizing magnetic field.

Within recent years, measuring devices have been proposed operative in response to the energy absorption occurring at the nuclear magnetic resonance frequency. From this absorption measurement, the relative proportion of an element in question can be determined because the total energy absorbed is a function of the number of nuclei present. Apparatus of this type can be used for the quantitative determination of any elementthe nucleus of which possesses angular momentum and magnetic moment, such as for example, hydrogen, helium, lithium, beryllium, boron, and nitrogen. Additionally, quantitative determination of various isotopes of elementscan also be made, because in many cases the different isotopes have different resonant frequencies.

The absorption phenomenon of nuclear magnetic resonance is also used to measure constituent proportions in various compounds. For example, moisture content measurements can be made in materials, such as tobacco or paper. In such a determination the Water content is States its not measured directly but, rather, indirectly by the amount 7 2,960,650 Patented Nov. 15, 1960 ICE radio-frequency current from a constant-current source is supplied to a parallel tuned circuit consisting of a coil and capacitor. The tank coil is placed within the uniform field of a permanent magnet so that the radio-frequency field is perpendicular to the magnetic field, and the material to be measured is placed within the coil.

The radio-frequency field, or the magnetic field, is modulated at a slow audio rate. When the radio-frequency and the magnetic fields satisfy the relation w 'yH, where w is the angular velocity of the radio-frequency field, H is the permanent magnetic field strength in gauss, and 'y is a constant dependent on the type of nucleus subjected to resonance, nuclear magnetic resonance occurs. In moisture measurements, the hydrogen nucleus is caused to resonate, and 7 equals 2.67 10' secgauss- The resulting nuclear resonance causes a decrease in the impedance of the tank circuit, and therefore a decrease in the voltage appearing across the tank circuit. For a given set of conditions the magnitude of this change in voltage is proportional to the amount of moisture prescut so that a quantitative measurement can be made.

The use of the nuclear magnetic resonance phenomenon in the measurement and control of the moisture content of various materials requires precise functioning of the measuring apparatus. In the event the required relationship between magnetic field intensity and the frequency of the radio-frequency field is not maintained, loss of resonance will render the measurement and control functions ineffective. Loss of resonance canoccur as a result of excessive variations in the intensity of the magnetic field, changes in the magnetic flux path, or through drift in the frequency of the radio-frequency field.

Accordingly, a principal object of this invention is to improve the reliability of nuclear magnetic resonance measuring and control devices by preventing loss of resonance with respect to a particular nucleus under test.

Another object is to provide improved nuclear magnetic resonance apparatus by controlling the frequency of the radio-frequency field in response to the intensity of the magnetic field.

Another object is to provide an electronic feedback control for attaining the foregoing objects that does not depend on the linear characteristics of vacuum tubes or transistors and therefore is stable for changes in tube characteristics, supply voltage changes and component replacement.

The invention may be associated with most of the magnet and coil assemblies employed in the prior art for generating the transverse magnetic and radio-frequency fields required for nuclear magnetic resonance measurements. In these equipments, a constant-current, radiofrequency oscillator is usually employed to energize the coil, and modulation coils may be coupled to the magnet so that the magnetic field is amplitude modulated at a slow rate. As the magnetic field periodically sweeps through the intensity value required to produce a condition of resonance for the frequency of radio-frequency field, the nuclei under test absorb energy.

A principal feature of this invention relates to a feedback network interconnecting a measurement readout circuit and the radio-frequency oscillator so that a con dition of nuclear resonance can be maintained through continuous satisfaction of the relationship w='yH. The feedback network causes the energizing frequency of the time relationship between the periodic occurrence of resonance and the start of each modulation cycle. In a preferred mode of operation, the radio frequency is set so that nuclear resonance occurs at the start of each modulation half cycle.

In order that all of the features for attaining the objects of this invention may be readily understood, reference is herein made to the drawings wherein:

Fig. 1 is a block diagram of nuclear resonance measuring apparatus employing the feedback control of this invention;

Fig. 2 is a graph showing a constant magnetic field modulated by a single alternating-current cycle;

Fig. 3 is a group of waveforms generated in the circuit of Fig. 1 and associated timewise with the modulation curve of Fig. 2;

Fig. 4 is a schematic circuit diagram of a difference detector satisfactory for use in the block diagram of Fig. 1.

Referring now to Fig. 1, material under moisture test is positioned in the interior of radio-frequency sampling coil 11, and is thereby subjected to a radiofrequency field parallel to the longitudinal axis of coil 11. Material 10 is also subjected to a transverse magnetic field developed in the gap between permanent magnets 12 and 13. Modulating coils 14 and envelop the pole ends of magnets 12 and 13, respectively, so that the otherwise steady magnetic field is amplitude modulated by the audio-frequency energy supplied from modulation source 16.

Capacitor 17 shunts coil 11 so that the combination 1117 forms a parallel resonant tank circuit connected to the output of constant-current, radio-frequency oscillator 18. The tank circuit is tuned to the oscillator frequency, and therefore a substantial radio-frequency voltage appears across the combination 1117. This voltage has a constant amplitude except during those periodic instances at which the output frequency of oscillator 18 and the modulated magnetic field generated by magnets 12 and 13 and modulation coils 14 and 15 satisfy the requirements for nuclear resonance.

During resonance, the moisture of material 10 absorbs energy from the radio-frequency field so as to periodically load coil 11. As is well known, the loading of a parallel resonant tank circuit lowers the Q of the tank, thereby reducing the parallel impedance and the voltage appearing across the tank. The periodic absorption of energy by material 10 amplitude modulates the radio frequency voltage appearing across tank circuit 11-17. The amplitude of this modulation component varies in accordance with the number of nuclei present to absorb energy from tank coil 11. In the case of moisture measurements, the hydrogen nucleus is caused to resonate, and the energy absorbed is proportional to the moisture content of the test sample. In the event material 10 is a moving web or cord, moisture variations in the sampled portion will cause corresponding changes in the energy absorption.

The voltage appearing across tank circuit 1117 is applied to the input of radio-frequency amplifier and detector 20. The output of unit is in turn applied to the input of audio-amplifier 21 which has an output connected to readout means 22. In a typical installation, readout means 22 may be an oscilloscope in which the vertical amplifier input terminals are connected to audio-amplifier 21, and the oscilloscope sweep is synchronized by the audio-frequency voltage generated from modulation source 16. This arrangement, therefore, produces a fixed pulse on the oscilloscope screen that varies in amplitude in accordance with any moisture variations in the material 10 under test.

Referring now to Figs. 2 and 3, the magnetic field strength in the gap between magnets 12 and 13 is plotted for a single cycle of the modulation signal 19 generated by source 16. The field intensity of the steady magnetic field is indicated by the ordinate H. This steady magnetic field is amplitude modulated at a low audio frequency having an amplitude that varies sinusoidally in accordance with the curve 19. The maximum and minimum amplitudes for the magnetic field are indicated by the ordinates H and H Two voltage pulses 23 (Fig. 3) are obtained at the output of audio amplifier 21 during each modulation cycle at times 10 and tO when w='yH. If the frequency of the radio-frequency field is increased slightly and the magnetic field is defined by curve 19, resonance will still occur, however, at times tA and tA At these times, the fields have the relationship w ='yH A slight decrease in radio frequency causes resonance to occur a times tB and 23 when the fields have the relationship w ='yH Resonance will not occur if the radio frequency exceeds the resonance frequency or if the frequency is less than the resonance frequency Y min The invention herein features a feedback network that maintains the output of oscillator 18 within the required resonance frequency range for variations in magnetic intensity deviating from curve 19. This feedback network operates on the principle of maintaining the time between voltage pulses appearing at the output of audio amplifier 21 to one-half of the period of the modulation cycle. Referring to the block diagram shown in Fig. l and the waveforms shown in Fig. 3, the feedback network operates in the following manner.

Frequency error is defined as the difference between the output frequency of radio-frequency oscillator 13 and the frequency at which w='yH. Direct-current error is defined as the direct-current voltage developed in the feedback network which is proportional to the frequency error.

The voltage pulse output 23 of the audio amplifier 21 is fed to a pulse shaper 24 which shapes, amplifies, and eliminates noise thereby generating sharp pulses 25 coincident with the audio output 23. The pulses 25 are gated in an electronic switching unit 26. This switch is supplied with a gating square wave 27 developed from the modulation voltage of source 16 and shifted in phase by phase shifter and amplifier unit 28.

The output of switch 26 therefore separates the two pulses 25 and generates one pulse 29 per modulation cycle. This pulse 29 is coincident with one of the pulses 25. The time at which pulse 29 occurs, with respect to the start of a modulation cycle, varies from /4 to A of the modulation period depending on the frequency error, and will equal /2 of the modulation period when there is no frequency error.

A frequency error voltage is generated by time discrimination means that is responsive to the time separation between actual pulse 29 and the time of a pulse 29 generated by the relationship w='yH. This discrimination means operates upon the principle of area detection and the comparison of two square waves. In particular, a monostable multivibrator 30 and a bistable multivibrator 31 are triggered at the start of each modulation cycle 19 by a trigger pulse 34 generated by trigger shaper 33.

The output of the monostable multivibrator 30 is a square wave 32 commencing at the start of each modulation cycle 19 and having a fixed time duration equal to one-half of the modulation period. Square wave 32 is employed as a reference waveform as is hereinafter set forth.

The bistable multivibrator 31 generates a square wave output 35 having the same amplitude as square wave 32 but with an opposite polarity. The duration of square wave 35 is dependent on the frequency error. In particular, the front edge of this square wave is coincident with the start of the modulation cycle 19 and the trailing edge is coincident with the gated pulse 29 present at the output of electronic switch 26. The outputs of the two multivibrators 3t) and 31 are inte-gr'atedand compared in a difference detector 36. A direct-current voltage proportional to the frequency error is applied to condoctor 37 as the output of the detector.

In order to achieve long term stability in the measuring device of this invention, the output of radio-frequency oscillator 18 is applied to a discriminator 38 that generates a direct-current voltage proportional to the frequency of the energy applied thereto. This direct-current voltage is added in amplifier 40 to the direct-current error voltage generated by the difference detector 35. The resulting output of adding amplifier 4015 applied to a reactance tube Within frequency control 41, thereby adjusting the frequency of the radio-frequency oscillator 18 so that pulses 23 assume a time spacing equal to one-half of the modulationcycle 19.

The feedback system herein described does not depend on the linear characteristics of vacuum tubes or resistors and is therefore stable for changes intube charac teristics, supply voltage changes, and component replacements.

Additionally, the basic feedback arrangement is applicable to alternate methods for achieving frequency control. For example, a direct current selsyn or syncroconverter may be driven by the direct-current error voltage so that a variable capacitor in the radio-frequency oscillator or-a potentiometer controlling the input voltage to a reactance tube may be appropriately controlled to effect the necessary frequency changes.

In another alternative arrangement,an amplifier driven to cut off and saturation by the modulating voltage '19 may be employed to generate the reference square wave 35 in lieu of the trigger shaper 34 and monostable multivibrator 30.

A preferred schematic circuit embodiment for difference detector 36 is shown in Fig. 4. In this circuit atrangement, the reference square wave 32 generated by monostable multivibrator 30 is applied between input terminal 50 and ground, and the variable time square wave 35 generated by bistable multivibrator 31 is applied between input terminal 51 and ground. The reference square wave 32 is applied through blocking capacitor 56 to a filter circuit including half-wave rectifier 52, capacitor 53, resistor 54 and capacitor 55. The variable time square wave 35 is applied through blocking capacitor 63 to a filter circuit including half-wave rectifier 60, capacitor 61, resistor 62 and capacitor 55.

It should be noted, that inasmuch as half-Wave rectifiers 52 and 60 are poled in opposite directions, the difference of the two voltages applied to the half-wave rectifiers is integrated in common capacitor 55. The voltage appearing across common capacitor 55 is applied to conductor 37 as the direct-current error output voltage. Resistors 64 and 65 provide discharge leakage paths for blocking capacitors 56 and 63.

It should be understood that the above described arrangements are merely illustrative of the principles of this invention and that numerous other modifications may be provided without departing from the scope of the invention.

What is claimed is:

1. Nuclear magnetic resonance measuring apparatus comprising variably adjustable means generating a radiofrequency field, means generating a steady magnetic field, means for cyclically amplitude modulating said steady field to define a modulation period such that the condition of nuclear resonance is periodically attained for the nucleus under test, means generating an output pulse for each occurrence of nuclear resonance, means generating a square wave pulse having a time length equal to and coinciding with the first one-half of said modulation period, means generating a second square wave pulse having a time length starting with the initiation of each modulation cycle and extending until the occurrence of resonance during a time span dependent upon the fre- 6 quency of said radio-frequency means and. defined by the middle onehalf of the period-of the modulation cycle, said square wave pulses both being of the same amplitude, and means responsive to the difference in the time lengths of said square wave pulses for variably adjusting the-frequency of said radio-frequency means to vary said time span until both of said square wave pulses are of equal time length.

2. Nuclear magnetic resonance measuring apparatus comprising variably adjustable means generating a radiofrequency field, means generating a steady magnetic field, means for cyclically amplitude modulating said steady field to define a modulation period such that the condition of nuclear resonance is periodically attained for the nucleus under test, means generating an output pulse for each occurrence of nuclear resonance, a monostable multivibrator generating a square wave pulse having a time length equal to and coinciding with the firstonehalf of said modulation period, a bistable multivibrator generating a square wave pulse having a time length starting with the initiation of .each modulation cycle and extending until the occurrence of resonance during a time span dependent upon the frequency of said radio-frequency means and defined by the middle oneahalf of the period of the modulation cycle, said square wave pulses both being of the same amplitude, and means including a difference detector responsive to the integrated differences in the time length of .said square wave pulses for variably adjusting the frequency of said radio-frequency means to vary said time span until both of said square wave pulses are of equal time length.

3. Nuclear magnetic resonance measuring apparatus comprising variably adjustable means generating a radiofrequency field, means generating a steady magnetic field, means for cyclically amplitude modulating said steady field to define a modulation period such that the condition of nuclear resonance is periodically attained for the nucleus under test, means generating an output pulse for each occurrence of nuclear resonance, means generating a first pulse having a time length equal to and coinciding with the first one-half of said modulation period, means generating a second pulse having a time length starting with the initiation of each modulation cycle and extending until the occurrence of resonance during a time span dependent upon the frequency of said radiofrequency means and defined by the middle onehalf of the period of the modulation cycle, and means responsive to the difference in the time lengths of said first and second pulses for variably adjusting the frequency of said radio-frequency means to vary said time span until both said first and second pulses are of equal time length.

4. Nuclear magnetic resonance measuring apparatus comprising variably adjustable means generating a radio- =frequency field, means generating a steady magnetic field, means for cyclically amplitude modulating said steady field to define a modulation period such that the condition of nuclear resonance is periodically attained for the nucleus under test, means generating an output pulse for each occurrence of nuclear resonance, a monostable multivibrator generating a first pulse having a time length equal to and coinciding with the first one-half of said modulation period, a bistable multivibrator generating a second pulse having a time length starting with the initiation of each modulation cycle and extending until the occurrence of resonance during a time span dependent upon the frequency of said radio-frequency means and defined by the middle one-half of the period of the modulation cycle, and means including a difference detector responsive to the integrated differences in the time lengths of said first and second pulses for variably adjusting the frequency of said radio-frequency means to vary said time span until both said firs-t and second pulses are of equal time length.

5. In nuclear magnetic resonance measuring apparatus having variably adjustable means generating a radiofrequency field, means generating a steady magnetic field, means for cyclically amplitude modulating said steady field to define a modulation period such that the condition of nuclear resonance is periodically attained for the nucleus under test, and means generating an output pulse for each occurrence of nuclear resonance, the improvement comprising means generating a first pulse having a time length varying in response to said modulation period, means generating a second pulse having a time length varying in response to shifts in the resonance time of a resonance pulse during modulation cycle, and means responsive to the difference in the time lengths of said first and second pulses for variably adjusting the frequency of said radio-frequency means until both of said first and second pulses assume a predetermined ratio of time lengths.

6. In nuclear magnetic resonance measuring apparatus having variably adjustable means gene-rating a radiofrequency field, means generating a steady magnetic field, means for cyclically amplitude modulating said steady field to define a modulation period such that the condition of nuclear resonance is periodically attained for the nucleus under test, and means generating an output pulse for each occurrence of nuclear resonance, the improvement comprising a monostable multivibrator generating a first pulse having a time length varying in Iv sponse to said modulation period, a bistable multivibrator generating a second pulse having a time length varying in response to shifts in the resonance time of a resonance pulse during each modulation cycle, and means including a difference detector response to the integrated differences in the time lengths of said first and second pulses for variably adjusting the frequency of said radiofrequency means until both said first and second pulses assume a predetermined ratio of time lengths.

7. In nuclear magnetic resonance measuring apparatus having variably adjustable means generating a radiofrequency field, means generating a steady magnetic field, means for cyclically amplitude modulating said steady field to define a modulation period such that the condition of nuclear resonance is periodically attained for the nucleus under test, and means generating an output pulse for each occurrence of nuclear resonance, the improvement comprising means generating a first pulse having a characteristic varying in response to said modulation period, means generating a second pulse having the same characteristic as said first pulse varying in response to shifts in the resonance time of a resonance pulse during each modulation cycle, and means responsive to a comparison in the above characteristics of said first and second pulses for variably adjusting the frequency of said radio-frequency means until both said first and second pulses assume a predetermined relationship in their characteristics.

Bloch Feb. 22, 1955 Mackey Nov. 27, 1956 

